1. Field of the Invention
The present invention is concerned with devices which generate ozone for numerous industrial applications such as oxidation of cyanide in electroplating waste, decolorization of dye stuffs, purification of refining waste water, disinfection of seawaters, treating paper for printing, water purification, odor control in sewage treatment and sterilization.
2. Related Application
This application contains subject matter common to U.S. Pat. No. 4,770,858 and entitled "Resilient Dielectric Electride for Corona Discharge Devices" and assigned to the same assignee.
3. Description of the Prior Art
Ozone generators commonly used today employ the corona discharge principle. This technique utilizes a high tension electrode and a ground electrode mounted in a spaced relation with a vitreous dielectric member covering one of the electrodes. The electrodes are connected to an electric power source to set up an electrostatic field or corona in the space between the dielectric material and one of the electrodes. An oxygen-containing gas (air or oxygen) is then passed through the space between the dielectric material and electrode.
Creation of the electric discharge field requires considerable expenditure of electrical energy. More than 80% of the electrical energy applied to the electric discharge field is converted to heat and, if this is not quickly removed from the electrostatic field, it will cause rapid decomposition of the product ozone back to oxygen. The rate of this reversal reaction increases rapidly above 35.degree. C. and is almost instantaneous at temperatures in the range of 200.degree. C. The dielectric member is generally in the form of a solid vitreous material such as glass, fused quartz or ceramic. Heat builds up in the vitreous dielectric material so that the material generally operates at temperatures approaching the critical 200.degree. to 250.degree. C. range. Proper cooling of the ozone generator is therefore critical to maintaining a practical operating efficiency and consistent yields of ozone. The temperature is usually maintained at approximately 100.degree. to 150.degree. C. by circulating air or oil along the back of the electrodes in order to cool the dielectric member. The vitreous materials are also very fragile requiring special handling both in manufacture as well as use.
In conventional ozone generators the inorganic materials are mixed with the dielectric material to create a homogeneous mass having an organic material distributed throughout the dielectric layer. High thermal conductivity is a requisite in the conventional ozone dielectric in order to allow the heat to flow through the dielectric to the heat sink which is in the form of a metal tube. Heat is carried away from the dielectric by passing a cooling liquid or gas through the interior of the electrode to maintain a low temperature on the electrode surface. In principal, a high thermal conductivity dielectric carries heat into the internal cooling medium more rapidly. In fact, cooling in this manner is ineffective because heat from exothermic reactions taking place in the corona region, causes highly localized surface heating which impairs ozone formation and causes formation of undesirable nitric oxides. Heat absorbed into the body of the highly conductive mixture of organic and inorganic materials only tends to accumulate in subsurface regions and contributes to the problem of temperature rise in the ionized air. Even cooling of the electrode or heat sink will not prevent heat build up in the ceramic material because of the temperature gradient which always exists through the cross section of a thermal conductor.
Prior art power supplies used for ozone generators have been of line frequency 60 HZ types and more recently higher frequency generators operating in the 800 to 1200 HZ frequency range. These power supplies apply a voltage in a sine wave form to the high voltage electrode. Others employ a circuit that forms a current wave form in a generally square wave configuration. Ozone generator electrodes have the general configuration of a capacitor and higher frequencies cause lower capacitive reactance, therefore, more current can be passed through a smaller discharge area. This more concentrated discharge, however, tends to cause increased heating of the electrode and a corresponding decrease in ozone production due to recombining of O.sub.3 into O.sub.2 at elevated temperatures.